I'd love to try this in the near future but im not 100% sure about STDs, should i ask an escort for a clear STD test beforehand? 

Why not, if you're diligent about your health and i think everyone connected with escort business should be aware of std's, you have every right to ask escort for tests. There are a lots of them who are willing to show their clear tests...

Of course they can. Almost all of STDs are spreadable through oral sex and that's why some escorts don't offer BBBJ but only covered. 

That's the risk you have to take if you want to nail a strange woman. And that's why i always keep seeing established escorts and stay away from prossies. Escort forums like escort rankings keep us informed and help us find reliable escorts. 

From that point of view STDs can spread even by sharing the glass with infected person? Am i wrong? 

Yes, but it depends on a lot more factors than spreading through an intercourse, that means, it's considerably harder to get STD through oral or using the same glass than through bareback sex.

Yes they can. Your mouth and throat can get infected and you can spread it to other people by kissing them or through oral sex spreading diseases on their genital area. Infected mouth and throat usually have no symptoms, though a small percentage may have sore throat etc. Risk of getting STDs through oral sex is few times lower than through genital or anal sex. Risk can be decreased by good dental health care, which means tooth decay or bleeding gums can increase risk of infection.

That's it, far lower chance of getting it through oral sex, that's what i've said. That's why most of the escorts offer bareback BJ...

How do precum affects chances of getting STDs through oral sex? 

There is a very low chance of transmitting disease through oral sex, unless you have a wound in your mouth which would expose it to your bloodstream.

I know a guy who's got a genital herpes through oral sex so be careful with whom you're having sex. 

What do you think about fingering and health risks it brings? 

It is possible but very unlikely. For example if you finger her and have a hangnail cut or something and her natural lubrication juice which contains STIs comes in contact with your blood, that's how it is transmitted usually.